T cell anergy is an important mechanism of peripheral tolerance that controls the development of immunopathology in experimental models of autoimmunity and transplantation, and is most likely operative in the context of clinical organ transplantation in humans. The immunologically important costimulatory receptors CD28 and CTLA-4 regulate in the induction of anergy, primarily through their effects on the production of T cell growth factors. T cell growth factors such as IL-2 are potent inhibitors of T cell anergy, and likewise exert a powerful influence on the outcome of allograft responses. Despite the wealth of information concerning the means by which growth factors promote the proliferation and survival of T cells, surprisingly little is known about how T cell growth factors oppose the induction of anergy, and precisely how such growth factors as IL-2 and IL-15 promote graft rejection during an alloimmune response is unclear. The goal of this proposal is to further dissect how signals from CD28, CTLA-4 and IL-2 control the decision between anergy and effector differentiation, and to what extent these signals operate through the regulation of cell division. The experiments described here should allow us to determine whether cell cycle progression is necessary or sufficient for IL-2-mediated anergy avoidance, and to identify which IL-2 receptor-coupled signaling pathways specifically contribute to anergy avoidance and effector T cell differentiation. The specific questions we will address are: 1) Do toleragenic stimuli induce anergy in primary T cells by inhibiting cell cycle progression?, 2) Which aspects of IL-2 receptor-coupled signal transduction are important for anergy avoidance and development of effector function in primary T cells?, and 3) How do growth factor-coupled signaling pathways and/or growth factor-driven cell division contribute to T cell effector function in vivo during an alloimmune response? A more thorough understanding of how CD28, CTLA-4 and IL-2 contribute to the induction of T cell anergy and the development of tolerance in a clinical transplantation setting may lead to therapeutic approaches with greater specificity and success.